Known analogue television tuners, for example in television receivers and video cassette recorders, are based on single conversion superheterodyne techniques in which a required radio frequency channel undergoes a single frequency conversion to an intermediate frequency before demodulation. In a typical application of such a tuner, the received bandwidth may be from about 50 to about 900 MHz or a substantial portion thereof and the intermediate frequency (IF) is typically in the region of 30 to 50 MHz, depending on which regional modulation standard is applied. Also, received channels may vary widely in received signal strength depending upon distance from a transmitter and this is referred to as the “far near effect”.
Because of the use of a single conversion technique and because the IF is substantially less than the transmitted band, the image frequency is within the transmitted band at a frequency which may be occupied by another channel. Because of the far near effect, this undesired channel may be of greater amplitude than the desired channel.
Groups of channels tend to be transmitted in clusters which occupy a smaller part of the received bandwidth. Although provision may be made for spatially separating such channels so that they are much less likely to interfere with each other, this is not always the case. In any case, the far near effect may result in strong potentially interfering channels occupying neighbouring frequencies to desired channels.
It is thus a requirement for such tuners to provide sufficient image rejection and cross modulation and intermodulation protection from interfering signals. In known television tuners, this is achieved by pre-filtering the received channels so that substantially only the desired channel is supplied to the frequency changing mixer. Provided potentially interfering signals are sufficiently attenuated, the desired channel can be selected and demodulated with acceptable performance. Such pre-filtering is provided by a plurality of filters between the mixer and the input to the tuner. However, because the tuner must be capable of tuning so as to select different channels, the filters must also be capable of tuning and must track the frequency of the local oscillator whose signal is also supplied to the mixer.
In practice, using low cost varactor diodes for performing the tuning function, a tuning range of only one or maybe 1.2 octaves can be achieved. It is therefore common practice to divide a tuner into three dependent channels for providing adequate frequency coverage of the whole received bandwidth within which desired channels may occur. Each of these independent channels requires its own tracking filters.
The local oscillator frequency also lies within the received bandwidth and it is therefore necessary to suppress re-radiation of the local oscillator signal from the tuner so as to prevent interference with other tuners. This is generally achieved by a further tracking filter for suppressing the local oscillator re-radiation.
A typical known type of single conversion television tuner thus contains three independent channels or bands, each of which has two sets of tracking filters which must provide sufficiently high Q to attenuate neighbouring potentially interfering channels (in the case of image filtering) and little or no attenuation to the desired channel (in the case of the local oscillator suppression filter). All of these filters must accurately track the frequency of the local oscillator.
The required tracking accuracy may be achieved by alignment during production of the television tuner or by some form of automatic alignment algorithm within the tuner. Production alignment requires an iterative adjustment during production whereas automatic alignment requires an iterative adjustment during use of a tuner. Both techniques involve significant costs.
FIG. 1 of the accompanying drawings illustrates the architecture of a typical known type of analogue television tuner, for example as disclosed in. “Colour Television” by Hutson, Shepherd and Brice, McGraw Hill, p104, ISBN 0-07-084199-3. The tuner comprises three channels or bands 1, 2, 3 of substantially identical arrangement or architecture but differing in the frequency ranges which they cover. The first channel 1 is designed to cover the band V1, the second channel 2 is designed to cover the band V3 and the third channel 3 is designed to cover the band U. Because the channels are substantially identical, only channel 1 will be described, channels 2 and 3 having the same structure. A tuner input 4 is connected to the inputs of the channels 1 to 3. In channel 1, the input is connected to a band limit filter 5 which limits the incoming bandwidth to the range covered by the channel 1. The band limit filter 5 can therefore have a fixed frequency response and, in particular, is not required to track the local oscillator frequency.
The output of the filter 5 is supplied to a single-tuned tracking filter 6 whose frequency is controlled by a varactor diode 7 in response to a tuning voltage supplied to an input 8. The filter 6 tracks the local oscillator frequency so as to suppress re-radiation of the local oscillator signal without substantially attenuating the desired channel.
The output of the filter 6 is supplied via an automatic gain control circuit 9 to a double-tuned tracking filter 10 which is tuned by varactor diodes 11 and 12 in accordance with the tuning voltage at the input 8. The filter 10 is an image reject filter for rejecting the image channel and has a bandwidth which is centred on the frequency of the desired channel with a −3 dB bandwidth marginally greater than the bandwidth of the desired channel. The output of the filter 10 is supplied to a mixer 13 which also receives the local oscillator signal from a local oscillator 14 which is tuned by a varactor diode 15 controlled by the control voltage at the input 8. The local oscillator frequency is generally equal to the radio frequency of the desired channel plus the intermediate frequency. The capacitances of the varactor diodes 7, 11, 12 and 15 vary in accordance with the tuning voltage and change the resonant frequencies of the networks with which they are associated. As is known, the resonant networks are aligned during manufacture such that, over the tuning voltage range, the centre frequencies of the various networks maintain the correct relative values or alignment.
This type of tuner has several disadvantages. For example, the radio frequency path between the input and the mixer of each channel or band 1, 2, 3 contains at least two tracking filters. The tracking filters must be aligned during manufacture or a dynamic alignment algorithm must be provided to maintain alignment during operation. Both techniques involve substantial cost. Also, in order to achieve the appropriate alignment, the varactor diodes 7, 11, 12, 15 must be well-matched. This increases component cost and causes difficulties in selecting a replacement diode if one of the diodes fails during use.